Steam generation

[youssefs]

I need to generate steam from flue gases of temperature 900 c with flow rate 15 ton/hr
How do i calculate it
Assuming final temperature for flue gases withing range 150 c

 

[LittleInch]

At a simple level it's just a massive equation once you know thermal heat capacity of your flue gas.

But 150C looks very ambitious.

You then need to know start temp of your boiler feed water and calculate steam production. Then allow something for losses.

This is a very simplistic approach, but then you've not given any details.

It might be a lot less steam than you think....

 

[youssefs]

At a simple level it's just a massive equation once you know thermal heat capacity of your flue gas.

But 150C looks very ambitious.

You then need to know start temp of your boiler feed water and calculate steam production. Then allow something for losses.

This is a very simplistic approach, but then you've not given any details.

It might be a lot less steam than you think....

The thermal heat capacity for flue gases around 78 kj/kg °c
After getting the heat content should i divide it over enthalpy of evaporation which is 2260 or something is missing ?

 

[LittleInch]

Evaporation of what?

You will need that for the steam for sure.

 

[Snickster]

You can calculate knowing the heat available from the flue gas and the inlet temperature of the condensate and latent heat of vaporization of the steam. You will need a heat recovery unit such as follows. Best thing is to contact a heat recovery unit supplier and they will advise.

 

[georgeverghese]

Generate steam at what pressure? Does steam product have to superheated - to what temp ?

 

[youssefs]

Generate steam at what pressure? Does steam product have to superheated - to what temp ?

It can be saturated steam i just need the process to know the quantity of heat can be used from flue gases at temp 900 °c and quantity of steam can be generated

 

[Snickster]

The heat available from the flue gas is

Q = m Cp dT

I don't calculate in SI units but in consistent units I believe this would be

Q = KJ/hr total heat available in flue gas
m = kg/hr mass flowrate of flue gas
Cp = KJ/(kg C) specific heat of flue gas
dT = change in temperature of flue gas in deg. C

If your steam has an enthalpy of vaporization of 2260 kJ/kg as you indicated above then total steam mass vaporized is:

m = Q/2260

where mass is in kg/hr of steam, Q is the heat available calculated above and 2260 is the latent heat of vaporization of the steam.

 

[youssefs]

The heat available from the flue gas is

Q = m Cp dT

I don't calculate in SI units but in consistent units I believe this would be

Q = KJ/hr total heat available in flue gas
m = kg/hr mass flowrate of flue gas
Cp = KJ/(kg C) specific heat of flue gas
dT = change in temperature of flue gas in deg. C

If your steam has an enthalpy of vaporization of 2260 kJ/kg as you indicated above then total steam mass vaporized is:

m = Q/2260

where mass is in kg/hr of steam, Q is the heat available calculated above and 2260 is the latent heat of vaporization of the steam.

When i calculated it this way the results were far too much to be possible
As i got the enthalpy of vaporization but didn't get the amount of heat needed to reach vaporization

 

[LittleInch]

The heat available from the flue gas is

Q = m Cp dT

I don't calculate in SI units but in consistent units I believe this would be

Q = KJ/hr total heat available in flue gas
m = kg/hr mass flowrate of flue gas
Cp = KJ/(kg C) specific heat of flue gas
dT = change in temperature of flue gas in deg. C

If your steam has an enthalpy of vaporization of 2260 kJ/kg as you indicated above then total steam mass vaporized is:

m = Q/2260

where mass is in kg/hr of steam, Q is the heat available calculated above and 2260 is the latent heat of vaporization of the steam.

The key is your dT which at 900 to 150 looks unfeasible without having a HUGE exchanger. Plus losses in terms of heat etc.

The high temperature end will be too hot for saturated steam and the low end needs to heat the incoming water.

See this for information. https://www.sciencedirect.com/science/article/pii/S2451904918300015

 

[EngrPaper]

The other limiting factor on the low temp available in your flue gas is the dew point. Flue gas often contains some nasties that you don't want condensing out and corroding your equipment. So if you have the composition of your flue gas, you need to figure out its dew point and remain a margin above that.

 

[MJCronin]

The other limiting factor on the low temp available in your flue gas is the dew point. Flue gas often contains some nasties that you don't want condensing out and corroding your equipment. So if you have the composition of your flue gas, you need to figure out its dew point and remain a margin above that.

The dewpoint factor is partiularly important . I believe you should investigate static economizers on boiler flues

Harvesting waste energy at the cold end of the process often involves using premium materials (or coatings) for HX tubes.

This will stongly affect the estimated costs... Review and investigate "condensing economizers"

There are startup, isolation and pressure relief issues with economizers also !!

 

[EdStainless]

I used to supply tubes for HRSGs.
If they were pushing for condensing at the cold end we would often be supplying four different alloys.
The hot end was often a CrMo steel and the second bank a plain boiler steel.
If the third section (economizer) was under 600F they would use 439, if it was hotter they would use 304.
The back pre-heat section where you get condensation of acids and water would be a super-ferritic, super-austenitic, or in some cases a Ni alloy.
It all depends.
You need an experienced designer.